The Dissolution Behaviour of Crystalline and Amorphous Albite in Acidic and Basic Environments at 25°C

Abstract

The dissolution behaviour of crystalline feldspar in the laboratory has been studied extensively for many years (Blnm and Stillings, 1995). From these studies, three well-known models evolved to describe the mechanisms of feldspar dissolution. The first model, which has generally been discredited, is based on diffusioncontrolled kinetics in which the feldspar dissolution rate is controlled by the diffusion of reaction products through an altered layer on the mineral surface. The second model is based on surface-reaction controlled kinetics in which reactions at the solid/solution interface control the rate of feldspar dissolution. A third model, which combines ideas from the other two models, is based on reactions at the surface and within the leached layer that control the overall dissolution behaviour of the feldspar (Hellmann, et al., 1990; Brantley and Stillings, 1996). The resistance to dissolution of some sodiumaluminosilicate glasses (including the albite composition) make them excellent matrices for the immobilization of radioactive fission products. Because the dissolution of crystalline albite is controlled by a surface-reaction mechanism, whereas the dissolution of albite glass is controlled by diffusion through an altered surface layer (Zellmer and White, 1986), comparisons of the kinetics of these two phases should shed light on the mechanism of dissolution. Few studies compare the dissolution kinetics of crystalline albite with amorphous albite and none of these studies were conducted at room temperature. Zellmer=s study indicates that the dissolution rate of albite glass is slightly faster than crystalline albite over a pH range of 2 -10 at 70~ (Zellmer and White, 1986). In this study, the leaching and dissolution behaviour of crystalline and amorphous albite was investigated in acid and base at 25~